Detoxification and resource recovery from extremely toxic metallurgical copper-cyanide wastewater via...

Cyanidation accounts for > 85% of gold production, producing alkaline cyanidation metallurgical wastewater containing extremely toxic and refractory copper cyanide complexes with high resource value. Traditional treatment of the wastewater is accompanied by generation of highly toxic cyanogen gas and hazardous copper-containing waste whose recovery needs secondary smelting. This study demonstrates a novel and straightforward strategy to recover metallic copper from the wastewater by ferrous-assisted zero-valent aluminum (Al⁰). In this strategy, Al⁰, as a strong reductant under alkaline conditions, completes the reductive decomplexation of copper cyanide complexes, generating metallic copper and free cyanide, while Fe²⁺ overcomes the thermodynamic barrier by sequestering the liberated free cyanide as low‑toxicity Fe(CN)₆^4-, thereby preventing the negative shift in the reduction potential of the remaining copper cyanide complexes and enabling Al⁰ to completely reduce all of them. Additionally, the coexisting Ca²⁺ and zinc cyanide complexes in the wastewater respectively interfere with the activity release of Al⁰ and the selective reduction of copper cyanide complexes, but these interferences can be resolved by pre-precipitation. Moreover, the large amount of low‑toxicity Fe(CN)₆^4- generated after Fe²⁺‑induced cyanide stabilization can be further recovered as Prussian blue via Fe²⁺ precipitation. Notably, this strategy enables direct recovery of 95.6% of copper from an actual wastewater (Cu ∼ 2.43 g/L), yielding metallic copper with a purity of 92% and Prussian blue with a purity of 53.4%, resulting in a net benefit of ∼ $17.7/m³ wastewater. The proposed approach provides a novel strategy to effectively realize detoxification and resource recovery of alkaline cyanidation metallurgical wastewater.